An optical fiber is an optical waveguide having a central core surrounded by an outer cladding. The refractive indices of the core and cladding are selected so that optical energy propagating in the optical fiber is well-guided by the fiber.
As is well known in the art, a single optical fiber may provide one or more propagation paths under certain conditions. These propagation paths are commonly referred to as the normal modes of a fiber, which may be conceptualized as independent optical paths through the fiber. Normal modes have unique electric field distribution patterns which remain unchanged, except for amplitude as the light propagates through the fiber. Additionally, each normal mode will propagate through the fiber at a unique propagation velocity.
The number of modes which may be supported by a particular optical fiber is determined by the wavelength of the light propagating therethrough. If the wavelength is greater than a "second-order mode cutoff" wavelength (i.e., the frequency of the light is less than a cutoff frequency), the fiber will support only a single mode. If the wavelength is less than cutoff (i.e., the frequency is greater than the cutoff frequency), the fiber will begin to support higher order modes. For wavelengths less than, but near cutoff, the fiber will support only the fundamental, or first-order mode, and the next, or second-order mode. As the wavelength is decreased, the fiber will support additional modes, for example, a third-order, fourth-order, etc.
Each of the normal modes (e.g., first-order, second-order, etc.) are orthogonal, that is, ordinarily, there is no coupling between the light in these modes. The orientation of the electric field vectors of the modes defines the polarization of the light in the mode, for example, linear vertical or linear horizontal. A more complete discussion of these modes, and their corresponding electric field patterns, will be provided below.
A number of devices have been constructed to utilize the orthogonality of the modes of an optical fiber to provide selective coupling between the modes. For example, copending U.S. patent application Ser. No. 884,871, entitled "Fiber Optic Modal Coupler," assigned to the assignee of this invention, describes a device which couples optical energy from the first-order mode to the second-order mode, and vice versa. U.S. patent application Ser. Nos. 820,513 and 909,503, both entitled "Fiber Optic Inter-Mode Coupling Single-Sideband Frequency Shifter," and both assigned to the assignee of this invention, disclose frequency shifters which couple optical energy from one propagation mode to another propagation mode while shifting the frequency of the optical energy. U.S. patent application Ser. No. 820,411, entitled "Fiber Optic Mode Selector," assigned to the assignee of the present invention, discloses a device which separates optical energy propagating in one of the first-order and second-order propagation modes from the other of the first-order and second-order propagation modes.